1. Field of the Invention
The present invention relates to communication networks and, in particular, to methods of admission control for inelastic applications such as Voice-over-Internet-Protocol (VoIP)-based telephony that generate packet traffic on a packet-switched communication network using the Internet Protocol (IP).
2. Description of the Related Art
VoIP-based applications traffic, as well as traffic sourced by other real-time applications such as videoconferencing, are among the fastest growing segments of traffic on IP-based networks. Unlike many legacy data applications, these real-time applications are often inelastic in the sense that they have stringent bandwidth or throughput requirements and cannot tolerate excessive delays or packet losses that may occur when a packet-switched network is congested. Thus, it is important for an inelastic application to know at the time of session setup if a path or paths between the session endpoints provide a required level of performance with respect to throughput, delays, and packet losses. If the application determines that the path performance is sufficient, it will continue to setup, or admit, the session; otherwise, the application will reject the session setup request. The process of determining, at the session setup time, whether or not the network can support the application transport requirements and then deciding to admit or reject the setup request is commonly referred to as admission control.
Admission control in IP-based networks may be performed using measurement-based admission control (MBAC) methods. An MBAC method makes admit/reject decisions for new VoIP calls based on the results of measuring transport performance characteristics of the communication paths between the respective source and destination nodes in the network and refuses the admission when such measurements cross certain thresholds. In the MBAC method, the measurements may be collected either from actual VoIP sessions, a method called passive measurement, or from synthetic test traffic injected between pre-selected nodes of the network, a method called active measurement. The advantage of passive measurements is that no additional load is placed on the network; however, fresh passive measurements are often not available. The advantage of active measurements is that, with appropriate test scheduling, fresh performance measurements can always be available; however, active measurements consume capacity that would otherwise be available for actual sessions, thus reducing the utility of the network.
There are two methods for collecting active measurements: continual and on-demand. In a continual method, active traffic is transmitted across paths at a sufficiently high frequency such that fresh performance data is always available. In an on-demand method, at session setup time, a short burst of synthetic traffic is transmitted across a path between session endpoints. The advantage of a continual method is that a post-dial delay is minimized, at the cost of an additional network load. The disadvantage of the continual method is that it does not scale to large networks. In a typical wide-area network (WAN), a volume of synthetic test traffic necessary to make accurate performance data immediately available for any and all paths between endpoints is excessive and may consume most, if not all, of the network capacity. The advantage of an on-demand method is that it scales, while the disadvantage of this method is that it adds to the post-dial delay and may have poor accuracy. Accuracy of the on-demand method may be improved by extending the duration of the burst; however, such means may lead to exceeding budgets for the post-dial delay.
Therefore, there is a need in the art for an improved method for measurement-based admission control for VoIP traffic in IP-based networks that will simultaneously minimize the volume of active measurement traffic and the post-dial delays while ensuring that fresh performance measurement data is always available.